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Re: [libvirt] [Qemu-devel] [RFC] block I/O throttling: how to enable in libvirt



On Fri, Sep 02, 2011 at 09:50:42AM +0100, Stefan Hajnoczi wrote:
>Date: Fri, 2 Sep 2011 09:50:42 +0100
>Message-ID: <CAJSP0QWc9OcOKxG3jGgYD3r5f=2Fqv3SNVcrARc2Tm0YmP+gyg mail gmail com>
>Subject: Re: [Qemu-devel] [RFC] block I/O throttling: how to enable in
> libvirt
>From: Stefan Hajnoczi <stefanha gmail com>
>To: Zhi Yong Wu <wuzhy linux vnet ibm com>
>Cc: libvir-list redhat com
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>
>On Fri, Sep 2, 2011 at 3:09 AM, Zhi Yong Wu <wuzhy linux vnet ibm com> wrote:
>> On Thu, Sep 01, 2011 at 09:11:49AM +0100, Stefan Hajnoczi wrote:
>>>Date: Thu, 1 Sep 2011 09:11:49 +0100
>>>From: Stefan Hajnoczi <stefanha gmail com>
>>>To: Zhi Yong Wu <wuzhy linux vnet ibm com>
>>>Message-ID: <20110901081149 GB14245 stefanha-thinkpad localdomain>
>>>References: <20110901050531 GB17963 f15 cn ibm com>
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>>>Subject: Re: [Qemu-devel] [RFC] block I/O throttling: how to enable in
>>>       libvirt
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>>>
>>>On Thu, Sep 01, 2011 at 01:05:31PM +0800, Zhi Yong Wu wrote:
>>>> On Wed, Aug 31, 2011 at 08:18:19AM +0100, Stefan Hajnoczi wrote:
>>>> >On Tue, Aug 30, 2011 at 2:46 PM, Adam Litke <agl us ibm com> wrote:
>>>> >> On Tue, Aug 30, 2011 at 09:53:33AM +0100, Stefan Hajnoczi wrote:
>>>> >>> I/O throttling can be applied independently to each -drive attached to
>>>> >>> a guest and supports throughput/iops limits.  For more information on
>>>> >>> this QEMU feature and a comparison with blkio-controller, see Ryan
>>>> >>> Harper's KVM Forum 2011 presentation:
>>>> >>
>>>> >>> http://www.linux-kvm.org/wiki/images/7/72/2011-forum-keep-a-limit-on-it-io-throttling-in-qemu.pdf
>>>> >>
>>>> >> From the presentation, it seems that both the cgroups method the the qemu method
>>>> >> offer comparable control (assuming a block device) so it might possible to apply
>>>> >> either method from the same API in a transparent manner.  Am I correct or are we
>>>> >> suggesting that the Qemu throttling approach should always be used for Qemu
>>>> >> domains?
>>>> >
>>>> >QEMU I/O throttling does not provide a proportional share mechanism.
>>>> >So you cannot assign weights to VMs and let them receive a fraction of
>>>> >the available disk time.  That is only supported by cgroups
>>>> >blkio-controller because it requires a global view which QEMU does not
>>>> >have.
>>>> >
>>>> >So I think the two are complementary:
>>>> >
>>>> >If proportional share should be used on a host block device, use
>>>> >cgroups blkio-controller.
>>>> >Otherwise use QEMU I/O throttling.
>>>> Stefan,
>>>>
>>>> Do you agree with introducing one new libvirt command blkiothrottle now?
>>>> If so, i will work on the code draft to make it work.
>>>
>>>No, I think that the blkiotune command should be extended to support
>>>QEMU I/O throttling.  This is not new functionality, we already have
>>>cgroups blkio-controller support today.  Therefore I think it makes
>>>sense to keep a unified interface instead of adding a new command.
>> QEMU I/O throttling currently don't support those options of blkiotune, such as --live, --config and --current.If those bps/iops settings are modified, it will immediately take effect.
>
>The --live, --config, and --current options are implemented inside
>libvirt and do not require hypervisor support.  Take a look at
>src/qemu/qemu_driver.c:qemuDomainSetBlkioParameters() to see how these
>options are implemented for blkiotune today.
I have understood this, thanks.

BTW: i have seen your comments against block I/O throttling patchset, and will respond next week.

Regards,

Zhi Yong Wu
>
>Stefan


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